The present invention relates to a method for growing diamond and cubic boron nitride crystals, and more particularly to a method for growing diamond and cubic boron nitride crystals having excellent mechanical strength and high regularity in particle size with high productivity, wherein seed crystals are efficiently placed in a regular pattern when diamond and cubic boron nitride are produced at ultra high pressure and high temperature.
In the production of diamond and cubic boron nitride crystals by means of a hydrostatic pressure method, an important factors for producing well-shaped crystals having few inclusions is to suppress the amount of generated crystal nuclei and to grow the crystals at a temperature and pressure in the vicinity of the phase equilibrium curve. Employment of seed crystals is an effective measure for suppressing the amount of generated crystal nuclei.
Even if the amount of crystal nuclei is suppressed, the temperature and pressure of a reaction portion must be maintained in the vicinity of the phase equilibrium curve in the stable region of the above-described high-pressure-phase substances. However, in an industrial ultra-high-pressure production apparatus, controlling the temperature and pressure of the reaction portion at the desired value is difficult. Therefore, even if seed crystals are employed, it is difficult to produce, with high productivity, diamond and cubic boron nitride crystals having a narrow grain size distribution, high percentage of crystal growth, and excellent mechanical characteristics, such as thermal toughness index and compressive fracture strength.
Japanese Patent Application Laid-Open (kokai) Nos. 68395/1986 and 68398/1986 disclose a method wherein seed crystals having a particular grain size are placed in a regular pattern on a supporting plate or a raw material plate (non-diamond carbon species and low-pressure-phase boron nitride), in order to control surrounding temperature and pressure of seed crystals serving as reaction portions. These publications disclose specific methods, including a method wherein seed crystals are placed into depressions formed on a supporting plate or a raw material plate.
In the above-described method for placing seed crystals in depressions formed on a plate, in order to place the seed crystals into the depressions efficiently, the seed crystals must be plated with metals (in the case of diamond crystal), or must be coated with alkali metals or nitrides (in the case of cubic boron nitride). However, seed crystals having a predetermined alloy composition are difficult to obtain by metal plating, and migration of impurities cannot be avoided in the plating process. In addition, the seed crystals coated with alkali metal or nitride are very susceptible to contamination. As a result, the produced crystals are of poor quality.
Meanwhile, since the depression is slightly larger than the seed crystal, the crystal may fall from the depression when the plates are stacked.
Furthermore, the above-described method requires a step for forming depressions on a supporting plate or a raw material plate and a step for plating seed crystals, and therefore, the method provides inefficient productivity.
The above-described publications also disclose a method for pressing seed crystals against a supporting plate or a raw material plate, but do not disclose a specific example for carrying out the method efficiently.
In order to overcome the above-described drawbacks, the present invention provides a method for producing diamond and cubic boron nitride crystals having excellent mechanical strength and high regularity in particle size with high productivity, wherein seed crystals are efficiently placed in a regular pattern on a supporting plate or a raw material plate (non-diamond carbon and low-pressure-phase boron nitride), and the temperature and pressure of a reaction portion are controlled at desired values in an industrial ultra-high-pressure production apparatus.